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Impedance

Discussion in 'Electronic Basics' started by Music Man, Mar 14, 2005.

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  1. Music Man

    Music Man Guest

    When a source(e.g.Mic or guitar)is added to load (Mic Input),how does the
    source become affected in transferring its signal?
    I believe that circuits work as voltage transfer rather than current
    transfer.
    What this exactly mean and why do we have things like high-F rolloffs when
    cables
    are too long in unbalanced systems?
    Is Mic output A/C seeing that it's a magnet? How is the voltge "understood"
    by the load input?

    Thanks
     
  2. The current delivered to the load has to flow
    thru the devices constituting the source. This
    normally produces a different output voltage
    than if the load was not present. In technical
    terms, the output impedance of the source and
    the load impedance together determine the
    signal delivered to the load.
    That can be a useful way to think about very
    low impedance sources. But with non-negligible
    output impedance and cable characteristics,
    that way of thinking can obscure important effects.
    The cable capacitance can load some sources
    excessively at higher frequency.
    Yes. (if it is a typical dynamic microphone)
    Sounds like a philosophical question. I would
    say the power delivered to the load causes
    things to happen that one hopes were desired
    effects. Then, anthropomorphically speaking,
    the input has been understood.
    You're welcome.
     
  3. John Fields

    John Fields Guest

    ---
    For a resistive source and load, model your source as a voltage source
    in series with a resistance and your load as a resistance to ground.
    Your circuit then becomes a simple voltage divider:

    Vs
    |
    [Rs]
    |
    +--->Vl
    |
    [Rl]
    |
    GND

    Where: Vs is the voltage source
    Rs is the source resistance
    Vl is the voltage appearing across the load
    Rl is the load resistance

    And can be described by:

    Vs * Rl
    Vl = --------- (1)
    Rs + Rl

    ---
    Charge gets transferred, But it's voltage that pushes it around.
    ---
    ---
    What what exactly mean?
    ---
    ---
    The longer the cable, the higher the capacitance, so the lower the
    capacitive reactance and the greater the loading on the source
    (generator) as frequency increases:


    Vs>---[Rs]--->>--+--[Rc]-->>--+---Vl
    | |
    [Cc] [Rl]
    | |
    GND>--------->>--+-------->>--+---0V

    Rc is the resistance of the cable connecting the source and the
    load,and Cc is the distributed cable capacitance shown as lumped, so
    Rc and Cc form a low pass filter with the reactance of Cc shunting
    high frequencies to ground.
     
  4. You may be only interested in the voltage from a source, but before
    that voltage can swing across cable capacitance, that capacitance must
    be charged twice a cycle by current from the source. The impedance of
    the source tells you how much current it can deliver before its output
    voltage gets loaded down by a given amount. For instance, if it is
    connected to a load impedance equal to its own, its output will fall
    to half of the unloaded value. High impedance sources work fine when
    connected with short wires (low capacitance) to high impedance
    amplifier inputs. But if you want to drive high cable capacitance
    without having that capacitance act as a low pass filter, you need to
    get the source impedance lower than the impedance of that
    capacitance. One way to do this is to put a step down transformer at
    the source to lower its voltage while increasing its current
    capability (lower its impedance). Amplifiers with so called "low
    impedance" inputs are just ones with higher voltage gain to compensate
    for the voltage step down.
     
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